Solubility and P-V-T relations and the growth of potassium titanyl phosphate

Journal of Crystal Growth 102 (1990) 427 433 North-Holland

427

SOLUBILITY AND P- V- T RELATIONS AND THE GROWTH OF POTASSIUM TITANYL PHOSPHATE R.A. LAUDISE and W.A. SUNDER AT&T Bell Laboratories, Murray Hill, New Jersey 07974, USA

and R.F. BELT and G. GASHUROV Airtron Division of Litton Industries, Morris Plains, New Jersey 07950, USA

Received 2 November 1989; manuscript received in final form 24 December 1989

KTP. potassium titanyl phosphate (KTiOPO

4) single crystals are the material of choice for frequency doubling Nd : YAG laser light to visible green. Solubility and P V T measurements were made at the conditions used in a new low temperature hydrothermal process for growth. P V T data show that in 2M K2HPO4 and 2M K2HPO4 +0.125M KNO3 saturated with KTP the pressures are not substantially different although they are greatly reduced from those of water. The data are used to show the temperature at which autoclaves fill with a single fluid phase as a function of various initial degrees of fill. This is of aid in crystal growth since better quality and reproducibility occur in the one fluid phase region Soluhilitv data in 2M K 2HP04 and 2M K2HPO4 + KPO3 show that KTP is probably incongruently saturating and not the stable phase except when excess KPO3 is present. These results explain the long equilibrium times and scatter in apparent solubilities in K2HPO4 and the formation of solid TiO~ inclusions during the initial stages of KTP growth. The phase equilibrium and solubility measurements suggested that growth in a slight excess of KPO1 would improve perfection by repressing Ti02 formation. This was expenmentally verified, but with some diminishing of growth rate on (011).

1. Introduction Single crystal potassium titanyl phosphate (KTP. KTiOPO4) is an important nonlinear optical material especially for the conversion of 1.06 p~mlaser radiation generated by the Na : YAG near infrared laser into 0.53 ~.tm visible green coherent light. It finds particular application in laser surgery where the visibility of the beam greatly facilitates the surgeon’s task [1]. Consequently commercial quantities of high quality large crystals are needed. Hydrothermal crystalization has been used to prepared crystals both in the laboratory and commercially [2]. Commercial processes originally used high pressure-high ternperature conditions (24 28 kpsi, 520 560 °C).The solutions from which KTP crystallizes corrode steel and non-ferrous alloys so that crystallization re0022-0248 90/$03.50

1990

quires growth in pressure balanced noble metal cans or in noble metal lined vessels. For efficient high pressure high temperature growth expensive large diameter internally heated vessels would be required. Some time ago we reported [3] that in 2M (molar) K2HPO4 mineralizer it was possible to crystallize KTP at practical rates at much reduces pressures and temperatures (10 kpsi and 375 425°C). Recently we scaled these conditions up to larger vessels and now routinely produce large crystals by this process [1]. Although we have reported phase equilibrium information on the system KPO4 Ti02 H20 [3], no solubility or P V T data have up to now been available. Cornparing P V T data, growth behavior and solubility is of great value in understanding and further improving growth and perfection. We have recently completed a study of the P V T relations

Elsevier Science Publishers B.V. (North-Holland)

428

R.A Laudise eta!.

Soluhil,a and P

and solubility of KTP under conditions relevant to its low temperature growth and report these results here. In this paper, we also report a procedure for TiO~reduction and perfection improvement based on our phase equilibrium studies.

2. Experimental Solubilities were measured in Pt capsules contamed in Tuttle type autoclaves [4].where pressure was provided by water pumped by a compressed air driven intensifier. The capsules were 3 cm length x 0.4 cm OD with a typical internal ~olume of 0.2 cm~.The technique has been described previously [5 7] so that only a brief outline is presented here. Appropriate mineralizer solutions (measured with a micro-syringe), and weighed KTP plates were added to the capsule first. All starting materials were reagent quality or comparable. The capsule was filled to an appropriate fraction if its free volume (percent of fill) with the desired mineralizer. Fill was chosen so as to approximately balance the external water pressure provided by the intensifier without either rupturing or crushing the capsule. Capsules were crimpled shut, welded and brought to the desired pressure with the intensifier. They were then heated to the desired temperature with a furnace ~hich could be slid into place over the autocla~e. Temperature gradients in the H20 pressure transmitting fluid were minimized by partly filling the top of the autoclave cavity with a steel rod. Following the run the autoclave was quenched in ice water and the weight loss of the plate was then taken as a measure of solubility. P V T measurements were made by the procedures we used for quartz saturated NaOH solutions [81and A1PO4 saturated H~PO4solutions [9], so only a brief description is required here. Since KTP saturated K2HPO4 solutions attack steel, a Pt lined vessel and Pt capillary take off of the sort used for A1P04 measurements was used. Pressure measurements were in 1 inch internal diameter x 2 inch internal Pt where lined Morey closure vessels, placed in a length furnace heat was supplied from a bottom hot plate and a top wrap around heater. Temperatures were measured by —

L

Trelaiioni and gronth of Ic TiOPO

4

externally strapped thermocouples. controlled by Leeds and Northrup Electromax II Proportional Controllers and regulated to make the autoclave isothermal. Pressure was measured using a Bourdon gauge calibrated on a dead weight tester. The pressure measuring autoclave was equipped with a special cover with a Pt capillary take off brazed to a high pressure tubing stainless steel capillary connected to the gauge [8,9]. This ensured that the hot corrosive solution contacted only noble metals. The autoclave was usually made isothermal within 1.5°C by manipulation of heater power. To ensure that the autoclave was isothermal an internal baffle such as is used in growth was not employed in pressure measurements.

3. Results and discussion 3.1. P V T relations

A series of P V T runs were made in 2M K2HPO4 and in 2M K,HPO4 saturated with KTP. Equilibrium was assured by only taking pressure measurements when pressure stabilized and always equilibrating for at least 24 h. In addition, pressures were identical whether the autoclave was heated or cooled. Fig. I shows the pressure as a function of temperature for 2M K2HPO4 for vanous initial fills. The pressure reading at point D. for example, was invariant from 6 h to 3 days and the result was independent of whether the previous point measured was C or E. Similar results were obtained at all points measured. Along A B F vapor + liquid coexist. For an initial fill of 84% * at B the autoclave fills completely with liquid. No abrupt discontinuity occurs at the critical temperature, C (374°C). The vapor phase is lost at H for 80% and at F for 75% fills. Fig. 2 shows analogous P V T relations for 2M K 7HP04 saturated with KTP. This solution also contains 0.125M KNOB which was found useful in growth experiments ensure kept 4 Asto can he that seen titanium pressureisalong oxidized to I i .

*

~ fill is the fraction of the free volume ~ 100 of the autoclave filled with solvent at room temperature.

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R.A. Laudise et al.

Solubiliry and P V T relations and growth ofKTIOPO

4

20,000

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200

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250

300

350

400

450

500

TEMPERATURE 1°C)

Fig. 1. Pressure for various fills as a function of temperature in 2M K2HPO4.

Fig. 2. Pressure for various fills as a function of temperature in 2M K2HPO4 +0.125M KNOB saturated with KTP.

the co-extensive curve where vapor and liquid are both present is not markedly different when the solution is saturated with KTP. The pressure of KTP saturated K2HPO4 is essentially unchanged from the pressure in K2HPO4. However, a slight reduction of 1000 psi occurs when KTP is added to 2M K2HPO4 at 80% fill in the one fluid phase region. Surprisingly, at 75% fill the addition of KTP to 2M K2HPO4 raises the pressure 20000 psi * * The pressure changes suggest, as has been shown for the Si02 [9] and A1PO4 [8] systems, that in dissolving KTP the number of ions is changed. The increase of pressure at 75% fill suggests a change in the dissolving reaction. As both fig. 1 and fig. 2 show, the pressure is substantially below that for pure water. The best region for growth is the lowest pressure region where the autoclave is in the all liquid range. This avoids the complications of solute

transport from liquid to vapor, low solubility in the vapor and boiling effects with probable concomittant imperfections. Figs. 3 and 4 derived from figs. 1 and 2 show the temperature at which the autoclave fills as a function of initial fill. These figures are important in choosing growth conditions. As can be seen at

We assume the addition of O.125M KNO3 in the KTP

Fig. 3. Temperature at which autoclave fills with fluid phase as

—

390

—

**

saturated case exerts a negligible effect,

370

-

330

-

310

-

~

-

TEMPERATURE AT WHICH AUTOCLAVE FILLS VS INITIAL FILL

—

LU

~

H 4

270 250 ____________________________________ 70 74 78 82 86 PERCENT FILL

a function of percent fill in 2M K,HPO4.

90

R..4. Lauthse era!

430

390

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TEMPERATURE AT WHICH AUTOCLAVE FILLS VS INITIAL FILL 2 MOLAR K2HPO4 + iZb MOLAR KNO3 +

370 350 LU 330

Soluhilit~’and P I~ Trelations and gronrh of KTiOPO

I

and quality improvement which we have observed in that range. For growth experiments when there

is a temperature gradient, as we have discussed in our pressure studies of AIPO4 [8] and SiO., [9], pressure is determined by a temperature weighted

-

H 4

310

-

290

-

0~

~

H 270

toward the lower temperature in the system. 3. Solubilitv

250 70

74

78

82

86

90

PERCENT FILL

Figs. 5a and Sb show the apparent solubility of KTP in 2M K~HPO4+ 0.125M KNO5 as a func-

Fig. 4 Temperature at which autoclave fills with fluid phase as a function of percent fill in 2M K2HPO4 +0 125M KNO saturated with KTP

tion of temperature at 10 and 22.5 kpsi respectively. Solubility is cxpressed as [(weight luss/cm~ solvent) X 100], i.e. % sol/cm~.The scatter is con-

siderably larger than for materials whose hydrothermal solubility we have measured previously.

the higher fills, the autoclave fills at a higher temperature when the solution is saturated with KTP. The effect is reversed at lower fills. From fig. 4 we see that the discontinuity in slope between 75% and 80% fill suggests a change in species which is tempting to associate with rate

We felt that perhaps this scatter was because KTP is not perfectly congruently saturating. That is KTiOPO4 does not crystallize from a solution where the ratio of constituents in solution is the same as the ratio in the solid, e.g. (KPO1/ TiO~)SOIUi(~l ~ 1.

16— KTP (KT1OPO4) SOLUBILITY 14

-

KTP (KTiOPO4) SOLUBILITY

o 2 M K2HPO4 +OI25MKNO3 10 (PSI

14

‘—‘12o

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2 M K2HPO4 +0.125M KNO3 22 5 kpsi

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____ ____________________

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450

500

550

600

TEMPERATURE (°C) 1 solvent)X 100] of KTP as a function of temperature TEMPERATURE in 2M(°C) K~HPO Fig. 5. Apparent % solubility [(weight loss cm 4+0.125M KNO.4 pressure: (a) 10 kpsi. (b) 22 5 kpsi.

R.A. Laudise et al.

Solubilitv andP V Trelations and growth ofKTiOPO

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431

H20 98 96 94

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E

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86 84

KPO3

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KTP~ II

KTP+1102

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82 p

80

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KTiOPO4 Fig. 6. Schematic of KPO3 Ti02 H20 phase diagram in 2M K2HPO4 at 10 kpsi, 500 and 600°C [21.

A schematic based on our previous determination [3] of the KPOI Ti02 diagram is shown in fig. 6. As can be seen the boundary KTP KTP + Ti02 (F H) lies close to and probably within the experimental error with which we could determine fig. 6 on the line where (KPO3/TiO2)50i~~,0~ 1 (H20 A). However, it is quite possible that at exactly (KPO3/TiO2)501~~~0~ = 1 (e.g. when initially dissolving solid KTP) the solubility is not congruent. The effect would be especially notable where a large volume of liquid must be saturated as in growth from nutrient in a large volume autoclave. Under such conditions some KPO3 dissolves resulting in some (TiO2)SOId remaining until the proper KPO3/T02 ratio is obtained following which KTP dissolves and recrystallizes. This effect could account for the inclusions of Ti02 sometimes seen in grown crystals (fig. 7). Fig. 7 shows a micrograph of rather well formed crystals included at the seed grown crystal interface of KTP grown in 2M K2HPO. Under these conditions, a light deposit of anatase Ti02 generally is seen on the top of the autoclave wall. X-ray powder diffraction identified these deposits as Ti02 so it is reasonable to assume the inclusions in grown KTP are also Ti02. To further investigate these effects, we measured the solubility of KTP as a function of KPO 3 as shown in fig. 9. Fig. 6 shows KPO3 in excess will assuredly bring a system into the solid —

Fig. 7. Ti02 inclusions at the seed-grown crystal interface in

KTP crystals grown in 2M K,HPO4 (larger crystallites

—

50

Hm). Marker represents 1 mm.

20 18 ‘-~‘

0

16

.

14

KTP (KTiOPO4) SOLUBILITY 550°C 22.5 kpsi 0 2 M K2HPO4 +0.125 M KNO3 • 2 M K2HPO4 +025 M KPO3

10

~

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-

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4 2

—-

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4I

6

8

10

DAYS

12

14

16

18

1 solvent) X 100]

Fig. 8. Apparent % solubility [(weight loss cm of KTP in 2M K,HPO4 +0.125M KNO1 and in 2M K2HPO4 + 0.25M KPO1 as a function of time at 550°C and 22.5 kpsi.

412

R.A. Laudue

et a!

Soluhiliti’ and P

12 KTP (KT1OPO4) SOLUBILITY a 550°C , 2MKHPO

‘-~‘

10

-

:

~-

~

2

4

excess of KPO1 in growth experiments. Ti02 formation was greatly reduced. However, there is some reduction in growth rate on (011).

+0125M KNO3

86

V T relations and growth of KTiOPO

4. Conclusions

-\

The pressure of KTP (potassium titanyl phos-

O O i

I

02

I

0.4

I

0.6

08

I

1.0

12

14

MOLARITYIM) OF KPO3 1 solvent) x 100] Fig. Apparent solubility [(weight loss/cm as a 9.function of %KPO in2MK 2HPO4+0.125M KNO1 at

KTP field. As fig. 9 shows, and as might be expected, common ion effects repress solubility and decrease data scatter. Incongruent saturation effects might account for the scatter in solubility in the absence of KPO.5 especially if leaching of KPO1 from solid KTP to established the proper ratio of [KPO.5/Ti02] in solution increases equilibrium time. Fig. 8 shows the dependence of apparent solubility on time in 2M K,HPO4 + 0.125M KNO~.As can be seen in the absence of KPO5, equilibrium time is in excess of the 2 day times generally seen for other hydrothermal materials [5 7]. Thus the apparent solubility data of fig. 5 while useful for crystal growth purposes should not be taken as equilibrium soluhilities. In addition, in the absence of KPO.5 the apparent solubility may be being measured at the edge or outside of the KTP-solid field. As a further test of this hypothesis we studied equilibrium times in K2HPO4 + KPO3. As can be seen in fig. 8 equilibrium times are greatly reduced when KPO1 is added lending credence to the argument that when (TiOJ/KPO~)~)/Ui~)flI we are outside the stability field of KTP. Thus in the absence of KPO~the first solid to precipitate during growth is Ti02 and the equilibrium time in dissolving is long and poorly reproducible while KPO3 is leaching from KTP solid solute. This argument was tested by using a slight

phate, KTiOPO4) saturated solutions in 2M K-IHPO4 has been measured under conditions appropriate for hydrothermal growth. The pressure is not too different from that of pure K2HPO4 solutions butthe is greatly reduced curve from the that pressure of pure H .,O. Along vapor pressure is virtually identical for 2M K2HPO4 and 2M K2HPO4 + 0.125M KNO3 saturated with KTP. Depending on fill KTP can either slightly raise or slightly lower the pressure relative to a solution of K2HPO4 in the all fluid phase region. The P V T data are used to slow the temperature at which the autoclave fills with fluid phase. These data are of use in choosing conditions for crystal growth. Solubility has been measured as a function of temperature, pressure and KPO~concentration in 2M K2HPO4 under conditions relevant to growth. In 2M K2HPO4 equilibrium times are long while in a slight excess of KPO3 they are like those of other hydrothermal materials. These results taken with a previously determined phase diagram support the conclusion that when [TiO2/KPO3]~01~~01 1 KTP is not the first material to crystallize. Instead Ti02 crystallizes until the liquid is enriched with KPO5 so as to enter the KTP field. This hypothesis explains why Ti02 inclusions sometimes are found in KTP and suggests a successful remedy for inclusion reduction the addition of a small quantity of KPO~to the growth solution. The addition of KPO) may lower the growth rate slightly.

—

Acknowledgements We would like to thank A.J. Caporaso for experimental assistance and R.L. Barns for the photograph of fig. 7.

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References

[51R.L.

[1] See, for instance, R.F. Belt, G. Gashurov and R.A. Laudise, SPIE Proc. 968 (1988) 100, and references contained therein. [21R.F. Belt, L. Drafall and K. Jensen, Presented at AACG-5 (5th Am. Assoc. for Crystal Growth Meeting), San Diego, CA, July 1981. [3] R.A. Laudise, R.J. Cava and A.J. Caporaso, J. Crystal Growth 74 (1986) 275. [41R. Roy and OF. Tuttle, Phys. Chem. Earth 1 (1956) 138.

[61 R.A. Laudise and E.D. Kolb, Am. Mineralogist 48 (1963) 64 (ZnO solubility). [7] R.A. Laudise and E.D. Koib and J.P. De Neufville, Am. Mineralogist 50 (1965) 382 (ZnS solubility). [8] ED. Koib, P.L. Key, R.A. Laudise and E.E. Simpson, Bell System Tech. J. 62 (1983) 639. [91E.D. Kolb and R.A. Laudise, J. Crystal Growth 56 (1982) 83.

Barns, R.A. Laudise and R.M. Shields, J. Phys. Chem. 67 (1963) 835 (A1203 solubility).